Wireless mobile voice and data communications have been achieved for some time now with macro cell base stations serving a large service area. Macro cells may be located on a dedicated tower or building top. Typically, each antenna serves one sector of an area surrounding the macro cell. Where more than one antenna is used for a given sector (e.g., receive diversity), antenna spacing may be adjusted for optimal spacing.
A newer trend involves adding small-cell base stations, especially in urban areas. These small cell stations are often used to increase capacity in an area already serviced by a macro cell. The equipment of the small-cell base stations is often installed on pre-existing objects of the city infrastructure. For example, small cell antennas may be mounted on a street utility pole using mounting structure. In such installations, antenna spacing is less readily adjustable, if at all.
It is often desired that the antenna system of the small cell uses a single transceiver coupled to multiple antennas, where the radiation patterns of the antennas are combined to form a quasi-omni directional radiation pattern for coverage of broad range of azimuth angles.
The antenna system located on a pole around a mounting structure may comprise a plurality of individual sector antennas (sometime called panel antennas) with main lobes oriented into different directions.
The individual antennas used in a small-cell antenna system are not necessarily designed for this purpose. For example, panel antennas designed for use in multi-sector base station applications may be used in the small-cell base station antenna system and configured into a quasi-omni single sector pattern. In the horizontal plane, the main lobe half-power beam-width of a sector antenna incorporated into the antenna system may be, for example, 60 degrees. In the real world, the beam-width of the sector antenna in use as well as the number of antennas placed on a pole may be not optimized specifically for creating a good quasi-omni radiation pattern. For example, the number of antennas may be dictated by various reasons—including economic reasons, and zoning regulations. As a result the radiation pattern of the small-cell station antenna system may be very far from optimal. To make the situation worse, the sector antennas may be mounted far from each other and the radiation pattern may have multiple maxima and nulls.
For example, assuming the sector antennas are identical, each antenna radiates the same power, and their phase centers are located on a circle diameter D, the overall radiation pattern of the antenna system will considerably depend on D; more precisely on D/λ. The series of radiation patterns shown in FIGS. 2 and 3 illustrate the effect of D/λ on radiation patterns. If 0<D/λ<1 the radiation pattern changes only slightly with Dλ; for D/λ>1, the radiation pattern is impacted much more.
The radiation pattern depends on D/λ=D*F/C, where F is frequency and C is speed of light. Therefore, the combined radiation pattern of the sector antennas is affected equally by increasing D and increasing frequency.
The radiation patterns of the individual sector antennas partially overlap. At large values of D/λ, several nulls and maxima may exist in the overlapping area. The pole diameter and the size of the antenna mounting structure can be big D/λ and removing nulls and maxima may be impossible. However, the location of maxima and nulls in the overlapping area can be controlled by phases of signals feeding the sector antennas.